The surface of an object may be coated with a polymer to protect the surface or to provide the surface with properties of the polymer coating. For example, coatings of synthetic polymers and natural biomolecules are applied to medical devices for a variety of reasons. In the case of catheters and guidewires, it is desirable to add a coating with a low coefficient of friction in the presence of water and a low tendency to form clots (thromboembolisms) in the presence of blood.
As discussed in U.S. Pat. No. 5,002,582, the disclosure of which is incorporated herein by reference, polymer molecules may be provided with latent reactive groups covalently bonded to them such that when the molecules are brought into bonding proximity with a surface (such as on a medical device), the latent reactive groups can be energized to form, via free active species generation, covalent bonds between these molecules and the surface. The latent reactive groups generate active species such as free radicals, nitrenes, carbenes, and excited states of ketones upon absorption of external electromagnetic or kinetic (thermal) energy.
The '582 patent describes a number of suitable latent reactive groups and some methods of applying polymers and other coatings to a surface using the latent reactive groups. Under one method, a solution containing a latent reactive molecule (e.g., a molecule having a latent reactive group) is applied to the surface. Thereafter, the desired polymer is brought into contact with, and is covalently bonded to, the latent reactive molecule, as to a reactive group different from the latent reactive group, to form a photocrosslinkable polymer system. The latent reactive groups may then be activated to cause the photocrosslinkable polymer system to covalently bond to the surface.
Under another method disclosed in the '582 patent, the surface is first coated with a solution of the molecules bearing latent reactive groups. UV light is applied to cause the molecules to covalently bond, through the latent reactive groups, to the surface. A solution containing the desired polymer is then applied to the surface, and the polymer bonds covalently to the photoactively treated surface. Further details of photoactive and thermally active covalent attachment of polymers may be found in U.S. Pat. Nos. 4,722,906; 4,973,493; and 4,979,959. The disclosures of these patents are incorporated herein by reference.
Bio-Metric Systems, Inc., markets a photoactive biocompatible coating technique under the trademark BioCoat. This technique first combines a photoactive binder and the desired coating material, such as a hydrophilic polymer, to create a photocrosslinkable polymer system. A solution of the photocrosslinkable polymer system is then applied to the medical device. The coating is dried, and the binder is cured with UV light to covalently bond the hydrophilic photocrosslinkable polymer system to the surface of the device.
Catheter guidewires are one example of the kind of medical device typically coated with a biocompatible material. In order to facilitate insertion of the guidewire into a patient and to minimize the threat of thromboembolisms, the metal core must be surrounded by a lubricious hydrophilic polymer coating. The coatings of prior art guidewires, however, are inadequate, for the reasons stated below.
The BioCoat process and the processes disclosed in U.S. Pat. Nos. 4,722,906; 4,973,493; 4,979,959; and 5,002,582 have not been used successfully to coat metal devices such as guidewires with hydrophilic polymers (despite suggestions to the contrary in those references) without first preparing or pretreating the surface of the devices, particularly when the coated metal devices are used in an aqueous environment. Two possible pretreatments have been proposed. Under one approach, the guidewire is prepared for subsequent coating by shrink-wrapping a polyethylene sleeve around the guidewire. The photocrosslinkable hydrophilic polymer system is then applied, dried and cured as described above. In another possible pretreatment method, the guidewire is prepared for subsequent coating by dipping the guidewire in a silane undercoat solution, then drying. The photocrosslinkable hydrophilic polymer system is then applied to the silane undercoating.
Other prior art methods of applying biocompatible coatings to metal devices use preparation methods that fall into the same two categories as the photoactive binder approaches mentioned above: (1) wet undercoat pretreatment methods and (2) solid sleeve undercoat pretreatment methods. For example, a wet undercoat method of applying a biocompatible, hydrophilic coating to a catheter or guidewire is disclosed in U.S. Pat. No. 5,135,516. The coating described in the '516 patent comprises an isocyanate primer, a lubricious binding component and an antithrombogenic component. The binding component is a hydrophilic, water-swellable, acid-containing polymer with quaternary ammonium cations bonded into the polymer layer. In one disclosed application method, a stainless steel guidewire is coated first with a primer solution of a 1% polyisocyanate in methylethylketone, then dried for 30 minutes. A topcoat of 1% poly(acrylic acid) and 0.5% of MYRJ 53 (nonionic surfactant) in dimethylformamide is then applied and dried for another 30 minutes. The resulting hydrophilic surface is then dipped first in a benzalkonium chloride solution, then dried and dipped in a heparin salt solution to complete the process.
U.S. Pat. No. 5,129,890 describes a solid sleeve pretreatment method for coating a metal guidewire with a lubricious coating. The guidewire coating method disclosed in the '890 patent interposes a polyurethane sleeve between the hydrophilic coating and the guidewire. The sleeve provides an attachment base for the hydrophilic coating.